Control system



y 5, 1954 G. A. KANE 2,679,138

CONTROL SYSTEM Filed May 19, 1957 2 Shee'ts-Sheet 1 4a Fig.1.

Fig. 2,

Characteristics in absence of F1 3. piston 39 and valve 40Characteristics of Error ccmp/ete system Displacement of valve 4/Displacement of piston l7 Garold A. Kane,

b8 Attorneg.

y 5, 1954 G. A. KANE 2,679,138

CONTROL SYSTEM Filed May 19, 1937 2 Sheets-Sheet 2 Inventor: B 3 GaroldA.Kafie1 b 5V mg Attorneg- Patented May 25,, 1954 UNITIZED: s rrnrzesCONTROL SYSTEM Garold A'K'gine, Scotia, N. Y., assignor to. GeneralElectriocompany, acorporation of'New York? v 18 Claims. 1 This inventionrelates. toacontrolsystems, more-- particularly to follow-up systems-forcontrolling a": driven object to" move into positional i agreement witha. pilot -devic.e,- and it-hashfor an vobject the provision. oflalsimplereliable andnimproved system of. this characten.

More s specifically, the. inventionv relates to. follow-up control.systems. in .whichthefollowing i or driven. obj e'ct isedriven-vby; anhydraulic, transmission dvice,..and.a further-object of theinvem tion.isthe provisionpf.improvedcmeans'for controlling the hydraulictransmission device to drive the driven object into positional agreementwith the pilot.- device in suoh'Ta manner that oscillation or -h'untingfofjtlieddrivenpbject about the position. of correspondence. or.agreement with the. pilot device is substantially eliminated.

In carrying the invention into effect .in one form thereof, anxhydraulicitransmissio'n device is provided for driving ,the driven.object, and meansfresponsive to. positionaldisagreement of the pilotdevice and "driven-object are provided forcontrolli nguth e transmissiondevice to drive tl1e..driv'en object i toward. positional. agreement.

with" the pi1ot.device together with means. re-

sponsive to the rate of change ofpositional disagreement of thepijlot'devi'ce and driven object foinmodifyingthecontrol of thehydraulic transmissiondevice' therebvto eliminate hunting.

For a better-'and more complete understandingunof lthe invention,reference should'now be had to the following;specification and to theaccompanyingdrawing in which'Figl is a simple, diagrammatic'al"representation of an embodimentof "th'e'inve'ntio'n, Figs. 2l'and'3-are charts of characteristic curves of toperating ,characteristicsservingto'explain the" manner in which the i invention functions to.eliminate ...hunting,. Figi l is" aisimple -diagrammatica1'representation ofa modification'oflth'esystem of Fig. 1. Fig 5 isadiagrammatical representation of the power tube"amplifier unitempioyedin' the system of Fig'r'l, andFi'gs: 6; 7; and 8 arediagrammatical representations of-operating positions of "lever I228at'vario'us points intlieoperation. In Figs. 6;7} and 8, the movementsof leverilztirom'its zerop'ositi'on are exeggerated in order to. 'illus(rate the operation. clearly.

Referring now to the drawing, an object str p for example as a gun it isto he drivenin a rate correspondencewitn a remotely located pilotdevieesuch'as .the'telescope-vi i. of awdirector unit for allpractical'motionseof the telescope by suitiii) able driving v-means.1illustrated-asanr hydraulic Y transmission device l2.

Although this: hydraulic transmission devices may. be of a anysuitabletype, itais :illustratedwasi. comprising--: a: variable J strokea hydraulic pump l 2a, an hydraulic motor element a I 2h-hconnected to:the pump by supply pipes A t and u I 4 through which it receives afluid :such as oil underipresr sure, one pipe serving at-any onetimer-asse sup plypipe and the otherhas-sa-Vreturnpipes- The, pump. isdriven by suitableadrivingi. means illuse treated as an alternatingcurrentinduetion motors. IZc which in turnvisssupplied:from-ta suitableesource representedbyt-thetthreehsnpply lines I 5.:4 The pumpisprovidedwvvithmeansr for varying-,4 the stroke thereofSincecthestructuresofwthen pump itself constitutes no. part.=0fiethespresentinventions-the stroke-.varyingt meansis not shown? inthedrawings Thisis stroke varying means,- howeven isactuated.-bvysuitablei. means. ,illusutrated a cylinder l fitand .a.piston= i 'Lslidably 1 mounted .therein.-. Theepiston is connectedtow thestroke varyingcmeans; by iineansiotsconnect-w ing-rod 18-.-

When-the piston-is in .the central or zero posittion in which it isillustrated- :.thestrokingqmeehe anism'is in its -zero:position-andnoloil issupplied to motor- I2-b. 1 lfcthe piston -is: moved= -upvvard fromits zero position l-the stroke varying mesh-aanisin isactuated. toincrease-the supplyiof oi1--. in.-, one direction to: the; motor and it.moved .downe Ward from" its zeroposition;-

the: rsupplyeof soil tothe motonis also increased but/the flow theopposite-direction. Thus-the speed andedirecticn. of motor I Eh Idepends-upon the: position of piston H.

The telescope! i is connected through-suitable gearing: to theerotor-rmember :of an-electrica1.-:- transmitting device l9.:-.This.l-transmittingnde-- vice comprises a rotor member. I9wpr0Videdwithw a single-phase! Winding- "(not shown); anda'statormember i 917 provided with La epolycircuitedistri outed: windingthata-is =physioally;-similar..-.to a polyphase windingyinvinduetiverelationsitrip with; the single-phase windings As *shown; thesing-1e-- phase Windingnis: supplied vfr-om'casuitablesouroe, 1 such. asone phase of-.-the-thr ee-.phasesupply line it to which it isconnectedttorconduotors-Z5;

Receiving device 2 his win all .res-pects.identical -fi in structurewithtransmittingdevice -l9; Theterminals of its distributednstatorzwinding rare? connected byymeansv of-rconductors 23E';%to;-ielec-=-trically corresponding;terminals: of thew-stators windingof thetransmitting :device-,-:- and wthe single-phase .rotorriwi-nding 10f;the rreceiving: ode-"r vice isconnected-by conductors-.24 stO:thergridai or input circuit of a tube amplifier 25 which in ture ofamplifier per se constitutes no part l of the invention, a detaileddescription is omitted. A substantially identical amplifier unit isdisclosed and its operation is described in United States Patent No.1,982,350, Mittag.

The output circuit of the amplifier is connected by means of conductors21 to the armature of a small direct current electric motor 28.

The transmitting and receiving devices i9 and 22 respectively act asrotary transformers. Voltages are induced in the distributed winding ofthe transmitter which are transmitted to the distributed windings of thereceiver as a result of which a voltage is induced in the single-phaserotor winding the magnitude and polarity of which depends upon therelative positions of the magnetic axes of the rotor windings of thetransmitting and receiving devices. That is to say when the rotors areninety electrical degrees out of correspondence with each other, thereis no voltage induced in the rotor winding of the receiving device. Ifthe rotor of the transmitting device is rotated ninety electricaldegrees from this zero position in either direction while the rotor ofthe receiver is held, maximum voltage is induced in the rotor winding ofthe receiver. If the rotation is in one direction, the induced voltagewill be of such a phase as to allow one pair of tubes e. g. pair 25a ofthe amplifier unit 25 to conduct current to the armature of motor 28 tocause it to rotate in one direction and if through diiferential gear 30to the rotor of receiver 22. This last connection comprises the gearing3|, shaft 32, gearing 33, shaft 35, cage member 33a, output gear 3%,gearing and rotor shaft of receiving device. Also, motor 28 is connectedthrough crank 33, and crank arm 31 to a system of linkages and valveswhich controls the actuation of the piston H which in turn actuates thestroke varying means of the hydraulic transmission device.

If the output shaft of the hydraulic transmission device is maintainedstationary, the rotor of the receiving device will be rotated by motor28 through gearing 52, 3!, shaft 32, gearing 33, shaft 34, cage member33a, gear 3%, and gearing 35 to follow the motion of the rotor of thetransmitting device as in any ordinary follow-up device. It is evident,therefore, that the total rotation of the motor 28 represents the lackof correspondence or the positional disagree ment between the pilotdevice and the output shaft of the hydraulic transmission device. Inother words, if the position of the rotor of the receiving device 22with respect to the rotor of the transmitting device is such that novoltage is induced in the secondary winding of the re ceiving device,and the motor 28 has not rotated from its zero position, then the objectdriven by the output shaft of the transmission device I2 is said to bein correspondence with the pilot device. It is further evident that ifthe transmitting device l9 and the driven object rotate exactlytogether, the rotor of the receiving device 22 is driven throughdifferential gearing 30 in correspondence with the rotor of thetransmitting device and consequently, the motor 28 will not be energizedand will not rotate. The total rotation of motor 28 therefore representsthe error in the system or in other words the positional disagreementbetween the pilot device and driven object. The only other source oferror is the electrical error existing between the transmitting deviceand the receiving device which produces the signal to the amplifier tocause rotation of the motor 28. This error is relatively so small thatit will be neglected and the error, as measured by the rotation of themotor, will be called the system error.

The system of valves and linkages operated by the crank arm 31 forcontrolling the stroke varying mechanism of the hydraulic transmissiondevice I2 comprises a cylinder 38 with a piston 39 movable therein, apilot valve 40 for controlling the admission of fluid pressure to thecylinder 33, and a pilot valve 4| controlled by the piston 33 and valve40 for in turn controlling the admission of fluid pressure to thecylinder !3. A link 42 serves to connect the valvestem 23 and theconnecting rod 44. Similarly, a link 4-5 pivoted at 45a serves toconnect the valve stem 43 with a link 41 that is pivotally connected toan intermediate point 42a of the link 52. It will also be noted that thecrank arm 3? is pivotally connected to the link 42 at the point 421).

Fluid pressure for the operation of the valves and 6! and the pistons 11and 39 is supplied from a pump 49 which receives oil from oil reservoir48 and which is driven by any suitable driving means, such as electricmotor 50. A pressure adjusting valve 5| is included in the dischargeline of the pump, and this valve functions to maintain a givenhydrostatic pressure,

on line 53 and to relieve excess pressure by allowing excess oil toreturn to the oil reservoir 48.

When any error exists in the system, that is to say when the telescopeII is moved out of correspondence with the gun II], the D.-C. motor 28is energized and rotates a proportional amount as explained in theforegoing. Rotation of the motor 28 moves the system of levers from itszero positions through the worm gearing 52 and the crank 36 and crankarm 31. The pivot point 4% between the crank arm 31 and the link 42 ismoved, accordingly necessitating movement of the valve stem 43 of valve40 or movement of the piston 39. The piston 39 cannot move because theoil from the cylinder would have to be pumped out against pressure sothe valve stem of valve 43 will move from the zero position in which itis illustrated. However, as

soon as this motion occurs, the valve ports are opened so that oil isadmitted to one side of the piston 39 and allowed to exhaust from theother side so that the piston 39 will move and continue to move as longas the valve 40 is open. This is accomplished as follows: Assume thatthe arm 3'5 moves upward, thus moving the valve stem 43 upward. Thepiston 39 acts as a term porary fulcrum for the lever 42. Oil underpressure from the oil pump is then admitted through 5: pipesea. andsfil- .into. =chamber 5'5,v out. :-pipe- 56 into the. .space beneath the.pistoniin cylinder. 38,"".thus forcing. the piston. 39. upward. The 0 11fr'bm..th'e=.space above the. piston. 39 :-is..al-lcwed to.-.flowthrough thepipe tilt-into chamber. iii and out .the valve ports-into.the-pipe. 59 :and. through-pipes. fiiiaand.6! .to. the pressure tank481'. Asthe. pistontdiisnmoved, the-pivot point 425 acts..as. a.fulcrumand the .piston 39.moves until ithe-valve stem. 43 is. restoredto zero, which cuts off .'the.oi1..supply tothe .cylinder 38 and thepiston. again comes towreste The operation: issuchthat if the varm 3I--ismoved at aaiven speed, the .valve 40 opens an amount. which allowsoil. to enter thecylinder-38 to. move the. piston; at .a rateproportional to that of theaarm"; Thus. itimav be saidfthat.the...disp-laceroent of: the .valvestem. 43 from its zero position is :adi-' rect.measure of the speed-.-of..movement of the-- arm 31,.and-hence;- is. a" measure of.- the rate "of changzeof the error. in.the system... In-.-thi.s con.- nection,-. it will be recalled-that .the:total-.-rota tion-of. .the -D.-C. motor fiarepresented the total.system.-error. Sincethe. arm .3! is;-.driven.:by

motorv 28, the displacement of the. *crank arm 1 .t'l-thereforaalsorepresents the-total error-.01- the :system,.andtherefora the rate-iofmovement ofthecrank arm 31 represents the rate of changeofIsystemer-ror. Sincethe piston 39 -mov.es..-at

a iratelproportional to='thathofthe .crankarm' Iiv 3lL'itithus-. becomesclear that-the displacement. of 2 the valve stem. 33 from its zeroposition is andirectrmeasure. of the speed of movement. of. thearm-.3l-and :also ofethe rate of'change of:

error in..the .system because\the speed ofimove-r' ment of thepiston-391s proportionalto the open-- ingotthe .valve 7 40-.

- It willv alsot-benoted .-.thatv -.the-'disp1acernent of l the. pivotpoint- 52a from :its=ze10 position".

pistontfi movesto close-the valve-and restore...

the-point 182a before it has-been 'displacedzvery" far. from its zeroposition. .So.-the-::position of. the. .point- 434% depends verydefinitely; upon the speed .of movement. of the-arm fi'tan'd therefore:upon the -rateof change of system error;-

Now for a very slow motion of the a-rrnf3l, ime; fo'rwery; low valuesof-the .rate ofchange-ortha error, the valve Aoremains very'nea-rly-on:zero-, andadisplacement of the valve sterncfit c1311)- portional..to the-displacementsoi ."theiarm:.3l.-' 23o.

occurshecause if the valve-to remains-:onizero;

the levertz tilts and movesthe-upper-aend' on; theclink t'l, whichmotionris transmitted-by" means of the lever 45, fulcrumlfia iandlink55'." toitheevalve 1H. motion of arm 3'!- isincreased, thusnecessitating thatvalve Ml open wider, a {)I'ODOITlOll.Gf"th9"di$placementof the stem of valve 40 is also trans-- mittedthrough the.stem..4l and lover 1&5 and" link 46 to the valve 4!... Thusit isevident that the displacement of the. valve stem of valve liconsists oftwocomponents, one dueentirelyto I the amount of movement of the arm 3?with valve- Mi "c n i r d st ion ry; and the second due Furthermore;-=if. the speed: of as to'the opening of valve ll.

is ameasure .of two quantities, (1); total system"; J

consideredastationary. Thereforey;theudisplaceement of the. valve 1 stem=46 of ;va-1ve-i4 l fromrzero; depends upon. the error-as givenfbygthedisplacement of the arm 31. and also on .the rate .ofchange of error as.measured by'the displacementzof the. valve 40. In. other words, theopening of: .valve Arise-function both of-the error andsof therateofchange of error of the-system; Thezinteir-i connecting links act"as-adiii'erential gear. aciding the two. components described in the:fore-s. going and transmitting the sum :thereof. ftorsthe valve-ti. Ifthe valve: ll is moved: fromtizero; in a downward-direction,then oil; isadmitted: under into'chamber Mb and through". pipe E3 into chamber {639.This of course causes; the piston. 61 to movedownward forcing oil. from:the chamber I so through pipe 62 .into chamber; Ma, and out throughpipes 64' and fil to theroil's. tank iBJ. As the piston ll moves fromitszero: or mid-position, it movesthe stroke'controlzmecha anisrn. of thehydraulic pump lilo; in? such manner thatthe quantityandtheidirectionnoir oil-flow to the hydraulic motor i217 is controlled;The speed of rotation of theoutput shaft ofrthei hydraulic transmissiondevice :is-directlypropore tional to the displacement-of piston iifromiits. zero or-inid-position. Consequently the E0061! eration of theoutput shaftis proportional .to the speed or" the piston which of courseis proportion'ali Since the opening of valve ll isa function'of bothntheerrortaridzthr: rateof change of error of the systeimasexplained: in.-the foregoing, it follows that the acceleration: of the output shaft ofthehydraulictransmissions is a function of the error and also of therateof change of erroraof-zthe :system';

In. the foregoing the terms errorfii andt'frate. of change of error areused interchangeably with. the terms fpositional disagreement"'and.r'ate of change of positional disagreement respectively.

From. the foregoing ,it will be understood that' for a. givenconstanterror, the output. shaft of the hydraulic transmission device willaoce'lera'te and. for the same given-magnitude of I error hut. one thatis still increasing,- theaccelerati'onsof-- "the-output shaft. will beproportionately larger."

and .will remain larger until thexspeed' of the'gun':

becomes equal to thespeed or the telescope At the inst-ant at which the:speed .of'the gun becomes: equal to the speedfof the .telescopeitheerror becomes constant momentarily, and. the rotor.

of the. receiver 221s. driven by output shafttzo throughthmdifierential. gearing 30" in.;corre.-=-

spondence with: .the rotor of the transmitter 19,

. andthereforerneither pair of tubes fifiaaor zfio of .the amplifier 25:is energized and motorfZB is r The on tputshaftoi the-trans stillaccelerating? When the therefore at-rest. mission .devicei2 is speed of.-the-:gun becomes greater th'an ithespeed comes negative; i. l e: itbegins to.?decrease:

speedor" .the telescopepit drives'throughthe dir ferential gearingstrand rotates the :rotor of the:- receiver 22 froinits positionofoorresp'ondence in such-ahirection that the :other .pairoi .tuoes ofrenderedmonducting and. the

the. amplifier 5 motor- 2t -caused to rotate inrth'e opposite' dir'ectionto move the arm '3'? downward; As-tbefoi'e,

the piston 39* acts temporarily "as a fulcrum'-.and

the left-hand end of linictliiisipulled downward; thereby opening valve.ME;

of. jch'ange Jot error .is .relatively. .large and the error relativelysmallfit will be seen that the component of downward displacement ofpivot point 4211. caused by downward displacement of valve stem 43 willbe greater than the component of upward displacement of pivot point 42acaused by upward displacement of pivot point 421) which is produced bythe positive error still remaining in the system.

As a result, pivot point 42a will move downward, and valve stem 46 willmove upward to close valve 4|. This stops the downward movement ofpiston I? and causes the transmission device I2 to operate at constantspeed. If the rate of change of error is still sufficiently great, i. e.if the error continues to decrease at a sufficient rate, the pivot point4211 will continue to move downward past its zero position, and thevalve stem 46 will continue to move upward past its zero position toopen the valve 4| and supply fluid under pressure to the space beneathpiston As a result, the speed of the transmission device l2 willdecrease. The various constants of the system may be adjusted so thatduring this period when the error is decreasing, the valve stem 46 ismaintained in its upward position sufficient to decrease the speed ofthe transmission at such a rate that by the time the error has reducedto zero, the speed of the driven object has decreased to a speed equalto that of the pilot device, i. e. telescope, and at that instant, valvestem 46 reaches zero and piston I7 stops so that constant speedoperation of the transmission results. Hence the driven object will bemaintained in exact correspondence with the pilot device.

If the error had been in the reverse direction initially, the operationwould have been similar but the reverse of the operation described inthe foregoing. Thus, whether the error and rate of change of error bothbe positive or both be negative, or whether one be positive and theother negative, the displacement of the pivot point 42a from its zeropoint and the displacement of valve stem 45 from its zero point are eachequal to the sum of components of displacement proportional to error andrate of change of error, and occupy positive or negative positions oneither side of their zero positions depending upon the relativemagnitude of these components.

The operation just described may be briefly summarized as follows:

If a given error exists, for any reason, the output shaft willaccelerate and will eventually reach a speed above that of the pilotdevice. Now a certain error exists but on account of the speed of theoutput shaft being greater than that of the pilot device, the error willbe decreasing. There will be a period, therefore, when a positive errorexists simultaneously with a negative rate of change of error.Consequently, the acceleration of the output shaft will be reduced tozero and the acceleration will also be reversed before the error itselfhas decreased to zero. Thus as the error reduces toward zero, the outputshaft decelerates to a speed equal to the speed of the pilot device, andinitial adjustments are so made that the speed of the output shaft ofthe transmission device becomes equal to the speed of the pilot deviceat the instant the error becomes zero, and thus stable operation isobtained at this speed with no further change in error. The overshootingor hunting is eliminated because it is not necessary tto reverse theerror in order to decelerate the output shaft to the proper speed.

The manner in which the piston 39 and valve 40 function to preventhunting will best be understood by first considering the operation ofthe system in the absence of these elements. It is to be recalled atthis point that the existence of an error between the pilot device andthe driven object controls the acceleration of the output shaft of thehydraulic transmission device and not the velocity. Now if the piston 39and valve 40 were eliminated, one of the two components of displacementof the valve 4| would also be eliminated, and the displacement of thevalve 4| would be exactly proportional to the displacement of the arm33' or directly proportional to the error. Now, if a certain errorexists, the valve 4| would be open a given amount and since thiscontrols the acceleration of the load, the latter would start to move ina direction to decrease the error and to close the valve 4|. The motionof the valve 4| would therefore be proportional to the error, as shownon curves 65 and 66 of Fig. 2, in which the curve 65 represents themotion of the valve 4| and the curve 66 represents the error. When theerror has become reduced to zero and the valve 4| has closed, the piston17 has, however, continued during this period to move in such a manneras represented by the curve 61 of Fig. 2. Thus, at the time the errorbecomes zero, the piston I! is not in its zero position andconsequently, the load will continue to rotate, driving through theposition of correspondence or positional agreement so that the errorreverses and continued oscillation, as shown by the dotted curves a, 66aand 67a, is established. In order to prevent these oscillations, i. e.in order to eliminate hunting, it is evident that the error, the openingof the valve M, and the position of the piston H must all reach zero atthe same instant.

Now consider the same initial conditions but with the devices 39 and 4tacting in the manner described in the foregoing. The conditions will beas shown by the curves 68, 69 and 10 of Fig. 3, in which the curve 68represents the displacement of valve 4|, the curve 69 represents theerror of the system, and the curve 10 represents the displacement of thepiston ll. The piston I! will start to move exactly as in the previouscase and the error will start to become zero. Valve 40 will then startto move so as to decrease the valve opening of valve 4 I, and hence,diminish the speed of piston l and thus thereby diminish theacceleration of the load in comparison with the condition shown in Fig.2. As the error decreases, valve 4| reaches the zero positionappreciably before the error becomes zero and actually reverses, asindicated by the curve 68. Thus before the error becomes zero, the valve4| is reversed and the piston l1 starts back to its zero position, asindicated by the curve 10.

The various system constants are initially adjusted so that when theerror has decreased to zero, the piston I! has also returned to zero andthe valve 4| returns to Zero. Thus the oscillations are damped out andstable operation without hunting is obtained.

In the modification of Fig. 4, the driven object, illustrated as a gunI00, is driven to follow the motion of a pilot device, such as telescopellll, by means of the hydraulic transmission device Hi2. This devicecomprises an hydraulic motor 92a supplied with a fluid such as oil froma pump I021) which in turn is driven at a speed which is preferablysubstantially constant by suitable means such as the alternatingcurrentmotor 13 which is supplied from the source H34. Connections H35and 06 complete a fluid circuit between 11 of the point D from its zeroposition is a function of the displacement of the point A whichdisplacement is proportional to system error, and is also a function ofthe displacement of point C which displacement is proportional to therate of movement of point A, or in other words, to the rate of change oferror. Thus point D corresponds to point 42a in Fig. 1 and itsdisplacement like the displacement of point 42a is proportional both tosystem error and rate of change of error. The

valve stem I I2a is directly connected to the point D of lever I26, andthe displacement of this valve stem from its zero position controls theflow of oil to the piston I08 which is connected to actuate the strokevarying means of the pump I021). Since the displacement of the point Dfrom zero is proportional to the error, and also to the rate of changeof error, the acceleration of the output shaft of the hydraulic motor I02a is proportional to the error and also to the rate of change of erroras explained in the description of the operation of the system ofFig. 1. With the proportionality properly adjusted, this results in astable follow-up mechanism. The operation of this system as thus fardescribed is substantially identical to the operation of the system ofFig. 1.

The operation may be summarized as follows: Assume the telescope II tobe moved rapidly in a direction such that the rotor IIIb of receiver II1 moves the stem of valve I23 to the left. Piston I will be moved tothe left a proportional amount in response to the admission of pressurefluid to space I25b. As a result, lever I26 will pivot about point B asa fulcrum since piston I I 5 has not yet started to move. Consequentlypoint A of lever I26 is moved to the left and point D is moved to theright, so that the lever I26 0ccupies a position as indicatedschematically in Fig. 6. Thus, the movement of point A to the leftrepresents the system error. This movement of lever I26 results inopening valve II2 so that pressure fluid is supplied through pipe I I2f, valve I and pipe I II to produce a rapid acceleration of strokepiston I08 which in turn results in rapid acceleration of the hydraulicmotor mm and its drive shaft I 01.

While the foregoing action is taking place, pressure fluid is admittedto chamber II4a as a result of the movement of point C to the right andthe opening of valve I I3. As a result, pivot B of lever I26 is moved tothe left, thereby moving lever I26 in a counterclockwise direction aboutpivot A, so that lever I26 approaches the position showndiagrammatically in Fig. '7. It will be noted that point D has beenmoved toward the left from its Fig. 6 position thereby partially closingvalve I I2 and decreasing the acceleration of motor I 02a and the gunI00 and receiver II 1 driven thereby. As the speed of the gun finallybecomes greater than the speed of the telescope, the gun begins toapproach a position of corre spondence with the telescope. In otherwords, the error begins to decrease. The stator Illa of the receiver II1 is now being driven at a greater speed than the rotor I I12) and inthe reverse direction. As a result, the rotor II1b begins to rotate inthe reverse direction in space so as to move the stem of valve I23 tothe right thereby admitting pressure fluid to chamber I25a. This causespiston I 25 to move to the right so as to rotate lever I26 in acounterclockwise direction about point B. As a result, point D moves tothe left and in so doing closes valve II2 thereby stopping the motion ofstroke piston I08 and stopping the acceleration of motor I02a and gunI00. However, since the speed of the gun is now greater than the speedof the telescope, the rotor of receiver I I1 will continue to rotate inthe reverse direction and continue to move point A to the right andpoint D to the left. By referring to Fig. '7 it will be noted that pointD will reach its zero position before point A reaches its Zero positionbecause the linkage again pivots about point B. As the motion continues,point D passes through its zero position and opens valve H2 in thereverse direction to cause pressure fluid to be supplied through pipe II2e to move stroke piston I08 in the reverse direction to deceleratemotor I02a as indicated diagrammatically in Fig. 8. It will be notedthat this action takes place before the error between the gun andtelescope becomes zero. In other words, deceleration of the gun beginsbefore it reaches a position of correspondence with the telescope andthe speed of the gun is decreased to the speed of the telescope whilethe gun is approaching correspondence with the telescope.

The movement of point D past its zero position to the left also movedpoint C past its zero position to the left thereby opening valve II3 toadmit pressure fluid to cylinder II4 to move piston II5 to the right.This causes lever I26 to move in a clockwise direction about point Atoward its zero position thereby closing valves H2 and H3 and stoppingthe reverse movement of stroke piston I08 and also stopping thedeceleration of the gun.

In the final or zero position of the linkage, point A and all the valvesare restored to their respective zero positions. The stroke piston hascome to a definite and stationary on stroke position and the motor I02adrives the gun at constant velocity.

When the telescope is either accelerated or decelerated from the speedat which it is being moved, a new error results and the sequence ofoperation described in the foregoing is repeated in a positive ornegative direction depending upon whether the initial motion of point Ais to the left or to the right.

In order to reduce the effect of lost motion in the linkage I26 whichwould introduce a certain time lag between the motion of the outputshaft I 01 and the responding motion of the point D, the shaft I59directly geared to output shaft I01, the friction member I60, and crankarm I6I connected to point D are provided. A small displacement ofoutput shaft I01 will rotate the stator of the receiving device II1which in turn causes a similar rotation of the rotor and a displacementof stem of valve I23 and a corresponding displacement of piston I 25.Now if lost motion exists in linkage I26, there will be no displacementof valve stem 212 until the displacement of piston I25 exceeds theamount of lost motion. Under these conditions a time lag would existbetween the motion of the output shaft I01 and the response of valvestem II2a. The shaft I59 is therefore rigidly geared to output shaft I01and drives the crank arm I6I through the friction clutch I60 to take upthe lost motion so that small movement of the output shaft I 01 isdirectly transmitted to valve stem I I2a even though some lost motionexists in linkage I26. After valve stem I I2a has been movedsufliciently to take up the lost motion, the friction device I60 merelyslips and subsequent motion of valve stem,

II2a results from motion of piston I25 as previously described.

The low speed receiving device II9 has a movv;=article-1:0ontactmre nherri 9;:connecte,d to. its; rotor membergslil 9b :50 :asmtoi rotate:rtherewith. -;.1Th=is E-IifiOVflblG contact memben is arranged-to:engage the othemofr'zt-he; pairs Contacts I30 stationary-contacts.I;3,Il,1-- I31.

.l are included-yin the=circuityof a; solenoid 132, and

.noid.- I. 33;.- as. illustrated. LThBcStSfiOI': member, of sthe lowspeedi-receivingrdevice H Q-is mechanicallycconnectedrtoitheaontputshaft: IB'L of the hydraulic transmissionandrelectrically .connectedgby'means oflrconductors @IM- ito:the:-:lowremedy-transmitting .gdevice fI wi-inithei director. Thus, the:motion of -thei :rotonmember; vI 4.9a. ot the :lowxspeed receivingdevice.; ,is,.-proportionalsto stheverror between .the gun andethetelescope. If;forsanyireasomihe .eerroreis greater than .say,:;3; therotorlzmember :ofirtheirlowspeed receiving ideviceiwillzibez officzeroJoyce corresponding amcuntz .and .Willccloselzone :ontheiotheri on the.pairs of; contacts r-Ifiil, 1:3 .I adespendingenponq .thedireotion oferror .betweemthc aIselesc opes-and gun, ,ther.eby;to energize onegof;the

. solenoids :I 32. or: I33. inturn pulls-the; stem .put {:shaft. .torotate. towards correspondence; at it:

vmaximum speed. eWhenr the *ontput-ishaft has irotatedto within, say;-2; -of correspondence, the

-.contacts -I 30,;or I3 I, aszthewase may .be, openwso that. thesolenoid I 32 or; 11:33. is.-.deene rgized and .the valve J35. isreturned .to-zero -,by. its centerin .springs 135a, ethus erestoringthew-control; of lithe 'ipiston I08 to the.- high-v-speed mechanisms.which loperatesin .the manner described in; the descrip-..ition.iofiethe operation .of :Fig. 1, .:to:bri r 1g,:the output shaftinto exact COIIQSDOIIdEIICBJisWith the .pilotrdevice, I 0 I.

InUorder-QGQ; preventa I large over-travel {of the gunwl 00- -when-,synch-ronizin z, the --lever .I 36 0p- ..eratedabv piston 121* andathecenterin v prin 4-37 operating-i onthesleeve I I 21): of thewalve 14 2 pare incorporated. wIt'will .-be noted: thatthe ful- .crum 436] -.ofllever. I36 ison, the. center line--01 ..the valve 143. .-1For smallerrors I which "involve "relatively-Jsmallvmotions-iof the, piston 125,-the ileverrz lfifi remainsstationarybecause of the -lost gmotiorrdevice. :I 38. However, 1 -for; larger 5811018 ,which :result in lar ermotions-of the piston ;I.-2 5, -thezvalve sleeve H2?) 'ismovedanwamountproppor tionalto .thermotion :of .thexpiston I25 .and-inthe samedirection as the displacement of'valve gstem' LL20... It is*thus apparent that even-for lar e motions of the piston I25, the :netrelative .motionbetween the'stem and sleeveof the valve 5HIsis/relatively small, except forthat component .of imotion of theevalvestem I Hot caused by-azdisplacement of thepoint C of the lever I25 fromits zero position: i. ..e..-.the componentproduced by s ratemfjchangeloferror. Now; if the'gun is apiproachingusynchronism at. high speed, atthe instantthe .lowi,v speed contacts open-to .retransfer rthescontroltoiwalve H2 and the high. speedor .fifin lmcontrol gsystem whichactuates "valve H2, themer-rori may beasrlarge as 2 -,-,but the. rateioflchangelof. errorwill bequite high so .thatthe left less than.the;'leftwardgdisplaceme t :OfulJDint Dsso .that ;.port I list :isppen:eta-the: lower li h handiedgeiofland 142g.

Now.:when the low seced contacts -openliinzare- .sponse todecreasing-error andqthepointiflzmones ,to- .the. right-fin respcnsetothe; negativemate 10f change of: error, a .very small-,movement; tithevalve stem .resul-ts..sln;-,closing-port. HZdnatsthe lower right-hand.edge andrquncoveringithefiport -I lzdat the lower leftehandzedee.:Inotherawords, ztheuvalveu-l l 2 been-reversed zwith consider- .ablyJess; motion .of. the valve :stemz. .1 iiai than would iordinarily berequired gtosmovefsthe; central land I I29, .across; the :port HMandthusrthe; r.esversalis accomplished morexrapidly than itiwould he inthe absence ofsleeve ;I,;I-; 2b and-iitsipneli lting mechanism.

The opening of the low-.zspeedrcontacts results in centeringthe .valve'.l35r sos thatgthezrewersal :of valve H2 supplies:oilzunderpressurezthrough pipe. III' to thezifi flcezbeneathg;the-zpistonslflfl.This causes piston A68 1- to .:.move; in thezvreverse direction ,to'decelerate-gthe rtransmissiongadevice and; gun. In 10131161";-W0151 S,fdeceleratiomhegins while .the GIIODiStStilLDOSiFbiS/B. :,Thus the systemanticipates the-point voficorrespondencemndsdecreases its :speedaccordingly. 'I-3his results; in synchronizationmhich is. muchlessabruptthan would otherwisebe attained.

In. order lto prevent overloading; of the hydraulictransmissionmnitcaused .by'the pperation ofrthe LGOHtl'DISvOR bY' ahigh-friction :load, av evice. to limit. :the possiblehorsepower.:transmittedfls zi ;cor-porated. This.deviceconsistsgoifthfipiste. 1:39 .movable; ina. cylinder. I430, whichyrcylinoler ispreferably formed;- in the same. cylinderiblock wi-th the cylinder :II39. The horsepower flimiting-gnech- .anism ;also 1 comprises the valve.I4 ,aalsogpr'ef- =erably formed ,in the icylinder; block with cy lindersI289 and -;I:4 0, and arrangedgin thezcohnections III), III between thecylindenjfliandzillhe -.control"mechan-ism. .Gentering springs .142 LandI43 are provided for-biasing, the piston." I;39.;a nd the valve-M Itowards their-zero positions. ;A link .ldd-iszconnectedto the. stem ofval-Ye 1M rand-als to. theconnectingsrod ofipiston I118. ;.This;link ;hMcooperatesswith. aw lost: motion {device Macon: the .connecting rodzofpiston 139. .Thedisplacement iOfEDiSlJOIlTI-B 9: fromitsszero orcenten-positionds .a :measuretoffthe hydraulic :pressure acrossathehydraulicimotor IMa. IT The speed of the hydraulic .motorimay :bemeasured by the displacement 'of the p'iston -flw whichactuatesthestroke varying mechanism of the pump "Hub-to-vary-the-speed ofthemotorI02a.- iConnectionsare-so made that if the :pistons I 08 moves upward'fromitszeropositionxthe atresulting pressurei-on. thezmotoritfli a..causes the apiston. vI139.1.:to:.:move downward. LThus .whenaz-certainlimiting'jcombination oi; speedzand metanpcninerof the.valve. I.I2,which-.-.with. these .315 pressure occurs;thelosttmotionineherdeviceeifi is taken up and the valve stem of valve MI will be movedupward from its zero position. This motion of the valve stem I4I stopsthe flow of oil to the piston I68 from all other sources, and oil underpressure is admitted to the piston H38 through the port I43 and out ofthe cylinder I09 through the port I4? of the valve I l! and back to thetank. This reduces the stroke of the pump and hence the speed of motorIllZc so that the output horsepower is reduced to theallowable value.The centering spring 143 then returns the valve stem of valve I iI toits zero position, and normal operation is resumed.

Usually a gun such as the gun I95 is mounted in such a place that itcannot rotate continuously, and accordingly, rigid mechanical stops areprovided. It is necessary, therefore, to incorporate a control devicethat will prevent the hydraulic transmission from driving the gun intothe mechanical stops under power. This device comprises the shaft I48geared to the gun, crank arm I49, levers I58 and I5I, lost motionmechanism I52, centering springs I53, and valve I54. The operation ofthis feature is as follows:

As the gun I!) approaches a stop, the point E of lever I50 is movedvertically, and the lever II pivoting about the point H takes the lostmotion out of mechanism I52 and raises the valve stem of valve I54. Atthis point in the operation, the piston I58 is in the dotted lineposition above its central or zero position. The valve I54 now blocksthe flow of oil through pipe III to the piston I08 from valves H2 andE35, and admits oil to the space above the piston I88 from the pressuretank through pipe I62 and the port 555 of the valve I 515, which forcesthe piston H18 downward towards its central or zero position so that thespeed of the transmission is reduced as the gun approaches themechanical stop, and just before the mechanical stop is reached, thespeed i is reduced to zero. If the action of the control is such as tocause the gun to rotate away from the mechanical stop, no restriction isimposed by the valve I54, as the flow of oil from the control valves H2or I may pass to the piston 508 through the check valves I5! and I58.

Although in accordance with the provisions of the patent statutes thisinvention is described as embodied in concrete form, it will beunderstood that the apparatus shown and described is merely illustrativeand that the invention is not limited thereto, since alterations andmodifications will readily suggest themselves to persons skilled in theart without departing from the true spirit of the invention or from thescope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1 A follow-up system for controlling a driven object to move intocorrespondence with a pilot device comprising an hydraulic transmissiondevice for driving said object, said hydraulic device having an outputshaft connected to said driven object and a device for controlling thespeed of said shaft, means responsive to positional disagreement of saidpilot device and driven object for actuating said control device tocause said driving means to drive said object toward correspondence withsaid pilot device, and means responsive to the rate of change of saidpositional disagreement for modifying the actuation of said controldevice thereby to eliminate hunting.

2. A follow-up system for controlling a driven object to move intocorrespondence with a pilot device comprising an hydraulic transmissionde- 'vice for driving said object, said hydraulic device having anoutput shaft connected to said driven object and a device forcontrolling the speed of said shaft, means responsive to positionaldisagreement of said pilot device and driven object for actuating saidcontrol device to cause said driving means to drive said object towardcorrespondence with said pilot device, and means responsive to thepositional disagreement of said pilot device and object and to the rateof change of said disagreement for modifying the actuation of saidcontrol device thereby to prevent hunting.

3. A follow-up system for controlling a driven object to move intopositional agreement with a pilot device comprising in combination, anhydraulic transmission device for driving said object, said hydraulicdevice comprising a variable stroke pump and an hydraulic motor suppliedtherefrom and having a control device for varying the stroke of saidpump, means responsive to positional disagreement of said pilot deviceand driven object for actuating said stroke control device to cause saidhydraulic transmission device to drive said object toward correspondencewith said pilot device and means responsive to the rate of change ofsaid positional disagreement for modifying the actuation of said strokecontrol device to prevent hunting.

4. A follow-up system for controlling a driven object to move intopositional agreement with a pilot device comprising in combination, anhydraulic transmission device for driving said object, said hydraulicdevice comprising a variable stroke pump and an hydraulic motor suppliedtherefrom and having a control device for varying the stroke of saidpump, means responsive to positional disagreement of said pilot deviceand driven object for actuating said stroke control device to cause saidhydraulic transmission device to drive said object toward correspondencewith said pilot device and means responsive to the positionaldisagreement and to the rate of change of said positional disagreementfor modifying the actuation of said stroke control device to preventhunting. 7

5. A follow-up control system for controlling a driven object to moveinto positional agreement with a pilot device comprising an hydraulictransmission device for driving said object, said transmission having avariable stroke hydraulic pump, an hydraulic motor supplied from saidpump, and means for varying the stroke of said pump to control theacceleration of said motor, actuating means for said stroke varyingmeans comprising a cylinder, a piston movable therein and connected tosaid stroke varying means, and means responsive to the rate of change ofpositional disagreement of said pilot device and object for actuatingsaid piston thereby to control the acceleration of said hydraulic motorin accordance with said rate of'change of positional disagreement.

A follow-up control system for controllin a driven object to move intopositional agreement with a pilot device comprisin an hydraulictransmission device for driving said object, said trans- 'mission havinga variable stroke hydraulic pump,

an hydraulic motor supplied from said pump, and means for varying thestroke of said pump to control the acceleration of said motor, actuatingmeans for said stroke varying means comprising a cylinder, a pistonmovable therein and connected to said stroke varying means, and meansresponsive to the positional disagreement of said pilot device anddriven object and to the rate of 17 change of said positionaldisagreement for actuating said piston thereby to vary the accelerationof said hydraulic motor in accordance with said positional disagreementand said rate of change.

'7. A follow-up system for controlling a driven object to move intopositional agreement with a pilot device comprising an hydraulictransmission device for driving said object, said transmission devicecomprising a variable stroke pump, an hydraulic motor supplied from saidpump and a device for controlling the stroke of said pump thereby tocontrol the acceleration of said motor, means for actuatin said strokevarying device comprising a cylinder and a piston movable therein andconnected to said stroke varying device, a valve for controlling thesupply of fluid pressure to said piston, and means responsive topositional disagreement of said pilot device and driven object foropening said valve to admit fluid under pressure tosaid cylinder to movesaid piston thereby to cause said motor to drive said object towardcorrespondence with said pilot device, and means responsive to the rateof change of said positional disagreement for modifying the openin ofsaid valve to vary the acceleration of said motor in accordance with therate of change or said positional disagreement.

8. A follow-up system for controlling a driven object to move intopositional agreement with a pilot device comprising an hydraulictransmission device for driving said object, said transmission devicecomprising a variable stroke hydraulic pump, an hydraulic motor suppliedtherefrom, and means for varyin the stroke of said pump, means foractuating said stroke varying means comprising a cylinder and a pistonmovable therein and connected to said stroke varying means, a valve forcontrolling the supply of fluid pressure to said cylinder to effectmovement of said piston, means responsive to positional disagreement ofsaid pilot device and driven object for controlling the opening of saidvalve, and means responsive to the rate of change of said positionaldisagreement for modifyin the opening of said valve, comprising a secondcylinder and piston movable therein, a second valve for controlling thesupply of fluid to said second cylinder, a connection between saidsecond valve and second piston and a connection between said connectionand said first valve.

9. A follow-up system for controlling a driven object to move intopositional agreement with a pilot device comprising in combination anhydraulic transmission device for driving said object, said transmissiondevice comprisin a variable stroke hydraulic pump, an hydraulic motorsupplied from said pump, and means for varying the stroke of said pump,means responsive to positional disagreement of said pilot device anddriven object for actuating said stroke varying means to control saidmotor to drive said object toward positional agreement with said pilotdevice, and means for modifying the actuation of said stroke varyingmeans in accordance with the rate of change of positional disagreementof said pilot device and object comprising a cylinder and a pistonmovable therein and connected to said stroke varying means and a valvefor controlling the admission of fluid pressure to said piston, a secondcylinder having a second piston movable therein and a second valve forcontrolling the admission of fluid pressure to said secand cylinder, alink connection between said second piston and second valve, and asecond link 18 connection between said first link and said first valve,and means responsive to positional disagreement of said pilot device anddriven object for controlling the opening of said second valve.

10. A follow-up system for controlling a driven object to move intopositional agreement with a pilot device comprisin in combination anhydraulic transmission device for driving said object, said transmissiondevice comprising a v iable stroke hydraulic pump, an hydraulic motorsupplied from said pump, and means for varying the stroke of said pump,means responsive to positional disagreement of said pilot device anddriven object for actuating said stroke varying means to control saidmotor to drive said object toward positional agreement with said pilotdevice, and means for modifying the actuation'of said stroke varyingmeans in accordance with the rate of change of positional disagreementof said pilot device and object comprising a cylinder and a pistonmovable therein and connected to said stroke varyin means and a valv forcontrolling the admission of fluid pressure to said piston. a secondcylinder having a second piston movable therein and a second valve forcontrolling the admission of fluid pressure to said second cylinder, alink connection between said second piston and second valve. and asecond link connection between said first link and said first valve, andan electric motor controlled by positional disagreement of said pilotdevice and driven object and connected to said first link for openingsaid second valve.

ll. A follow-up system for controlling a driven object to move incorrespondence with a pilot device comprising in combination anhydraulic transmission device for drivingsaid object, said hydraulicdevice having an output shaft connected to said driven object, and adevice for controlling the speed of said shaft, means responsive topositional disagreement of said pilot device and driven object foractuating said speed control device to cause said driving means to drivesaid object toward correspondence with said pilot device, meansresponsive to the rate of change of said. positional disagreement formodifying the actuation of said control device thereby to eliminatehunting, and means responsive to positional disagreement greater than apredetermined value for rendering said rate of change responsive meansinactive and actuating said speed control device to drive said objecttoward correspondence with said pilot device at maximum speed.

12. A follow-up system for controlling a driven object to move intocorrespondence with a pilot device comprising in combination anhydraulic transmission device for driving said object, said hydraulicdevice havin an output shaft connected to said driven object and meansfor controlling the speed of said shaft, a cylinder having a pistonmovable therein and connected to said speed control means, connectionsfrom a source of fluid pressure to said cylinder, a valve in saidconnections for controlling the movement of said piston, meansresponsive to positional disagreement of said pilot device and drivenobject for actuating said valve to control said hydraulic device todrive said object toward correspondence with said pilot device, meansresponsive to the rate of change of said disagreement for modifying theaction of said valve to eliminate hunting, a second valve included insaid connections between. said flrst valve and speed control device andmeans responsive to positional disagreement greater than a predeterminedvalue for actuating said second valve to render said first valveinactive and for actuating said speed control device to cause saidhydraulic device to drive said object toward correspondence at maximumspeed.

13. A follow-up system for controlling a driven object to move intocorrespondence with a pilot device comprising in combination, anhydraulic transmission device for driving said obiect, said hydraulicdevice having an output shaft connected to said driven object and meansfor controlling the speed of said shaft, a high speed motiontransmission system res onsive to the positional disagreement of saidpilot device and obiect for controlling said speed control means todrive said object toward correspondence with said pilot device, meansres onsive to therate of change of said disagreement for modifying theaction of said speed control means to eliminate hunting, and a low speedmotion transmission system responsive to a predetermined value of saidpositional disagreement and a switchin device actuated thereby forrendering said high speed transmission system inactive and forcontrolling said speed control means to drive said object at maximumspeed.

14. A follow-up system for controllin a driven object to move intopositional agreement with a pilot device comprising an hydraulictransmission device for driving said object, said transmission devicehaving an output shaft, and means for controlling the speed of saidshaft. means responsive to positional disagreement of said pilot deviceand driven obiect for actuatin said speed control means to control saidhydraulic transmission device to drive said object toward correspondencewith said pilot device, and means for limiting the power output of saidtransmission device to a predetermined maximum value.

15. A follow-up system comprising in combination a pilot device, adriven object, an hydraulic transmission for driving said object, saidtransmission device having an output shaft connected to said obiect andmeans for controllin the speed of said shaft, a cylinder, a pistonmovable in said cylinder and connected to said speed controlling means,means responsive to positional disagreement of said pilot device anddriven object for controlling the supply of fluid pressure to saidcylinder to eii'ect movement of said piston thereby to actuate saidspeed control means to control said hydraulic transmission device todrive said object toward correspondence with said pilot device, andmeans for controlling the supply of fluid to said cylinder to limit thepower output of said transmission device to a predetermined maximumvalue.

16. A follow-up system comprising in combination a pilot device, adriven object, an hydraulic transmission device comprising a powerdriven fluid pump, a fluid motor supplied from said pump, fluid supplyand return connections between said pump and motor, and means forcontrollingsaid pump to control the speed of said motor, a cylinder, apiston movable in said cylinder connections from a source of fluidpressure to said cylinder, means responsive to positional disagreementof said pilot device and driven object for controlling the supply offluid pressure through said connections to effect movement of saidpiston thereby to actuate said speed controlling means to control saiddrivin means to drive said object into correspondence with said pilotdevice, and means for limiting the power output of said transmissiondevice comprising a valve in the connections from said source to saidcylinder and a device responsive to the fluid pressure supplied to saidpump, and cooperating with said piston to control said valve.

17. A follow-up system for controlling an object to move into positionalagreement with a pilot device comprising in combination an hydraulictransmission device for driving said object, a device for controllingthe speed and direction of rotation of said transmission, a cylinderhaving a piston movable therein for actuating said control device,connections from said cylinder to a source of fluid pressure, a valve insaid connections, means responsive to positional disagreement of saidpilot device and driven object for actuating said valve to control saidtransmission device to drive said object toward correspondence with saidpilot device, a second valve in said connections between said firstvalve and said piston, and means operable in limiting positions of saidobject for controlling said second valve to interrupt the supply offluid from said first valve to said piston and for supplying fluid tosaid piston to actuate said control device to stop the driving action ofsaid transmission device.

18. A follow-up system for controlling an object to move into positionalagreement with a pilot device comprising in combination an hy-- draulictransmission device for driving said object, a device for controllingthe speed and direction of rotation of said transmission, a cylinderhaving a piston movable therein for actuating said control device,connections from said cylinder to a source of fluid pressure, a valve insaid connections, means responsive to positional disagreement of saidpilot device and driven object for operating said valve to control saidtransmission device to drive said object toward correspondence with saidpilot device, a second valve in said connections between said firstvalve and;

said piston, means operable in limiting positions of said object forcontrolling said second valve to interrupt the supply of fluid from saidfirstvalve to said piston and for supplying fluid to said piston toactuate said control device to stop the driving action of saidtransmission device. and check valves in said connections providing foractuation of said piston by said positional disagreement responsivemeans to cause said transmission device to drive said object away fromsaid limiting position.

References Cited in the file of this patent UNITED STATES PATENTS

